Why Would Someone Ever Declare More Than One Card?
One reason a character may wish to declare drawing more than one card at a time is action economy. In the case where a character is already in combat when drawing a card, or if the act of drawing a card causes them to enter a combat scenario a player must consider their action economy concerning further draws. As it is not explicitly stated in the wording of the Deck of Many Things, depending on how your DM rules it, choosing to interact with the deck and draw cards would likely either require the use of your Action or your free object interaction. Since a player only has one of either per turn, this could affect the use of the Deck of Many Things.
A player could in one turn declare to take more than one card and have all effects occur on that turn, as opposed to having to wait for their next turn to draw an additional card. Even if a DM were to rule that you could only physically draw one card in your turn of combat the specification that "If you fail to draw the chosen number, the remaining number of cards fly from the deck on their own and take Effect all at once" would require all effects to occur on that turn.
